The Affordable Care Act (ACA) has substantially increased the number of Americans with public and private health insurance coverage. The Assistant Secretary for Planning and Evaluation (ASPE) at the US Department of Health and Human Services estimates that the ACA has resulted in 20 million additional nonelderly adults gaining coverage between the law’s enactment and February 2016. This estimate is likely overstated. Government surveys’ estimates of the number of people who gained coverage between December 2013 and December 2015 vary by 20 percent. Moreover, while the ASPE estimates that the ACA increased the number of young adult dependents with insurance coverage between 2010 and 2013 by 2.3 million, data from government surveys indicate that 1.2 million fewer dependent children had private coverage in 2013 than in 2010, offsetting half the gain in coverage among older dependents. Coverage gains have nonetheless been significant, with most of the increase coming from enrollment surges in Medicaid and the Children’s Health Insurance programs. But a substantial proportion of those who have enrolled in these public programs since 2014 met eligibility standards that predated the ACA. This increase in public coverage may have crowded out private coverage, although further study is needed to determine the existence and magnitude of this effect.